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Remove large fill factor support from dynahash.c. 

Since ancient times we have had support for a fill factor (maximum load
factor) to be set for a dynahash hash table, but:

1.  It was an integer, whereas for in-memory hash tables interesting
load factor targets are probably somewhere near the 0.75-1.0 range.

2.  It was implemented in a way that performed an expensive division
operation that regularly showed up in profiles.

3.  We are not aware of anyone ever having used a non-default value.

Therefore, remove support, effectively fixing it at 1.

Author: Jakub Wartak <Jakub.Wartak@tomtom.com>
Reviewed-by: Alvaro Herrera <alvherre@2ndquadrant.com>
Reviewed-by: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Reviewed-by: David Rowley <dgrowleyml@gmail.com>
Discussion: https://postgr.es/m/VI1PR0701MB696044FC35013A96FECC7AC8F62D0%40VI1PR0701MB6960.eurprd07.prod.outlook.com
This commit is contained in:
Thomas Munro 2020-09-19 11:28:34 +12:00
parent 06a7c3154f
commit be0a666665
2 changed files with 6 additions and 16 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

20
src/backend/utils/hash/dynahash.c
View File

@ -122,7 +122,6 @@
#define DEF_SEGSIZE 256
#define DEF_SEGSIZE_SHIFT 8 /* must be log2(DEF_SEGSIZE) */
#define DEF_DIRSIZE 256
#define DEF_FFACTOR 1 /* default fill factor */
/* Number of freelists to be used for a partitioned hash table. */
#define NUM_FREELISTS 32
@ -191,7 +190,6 @@ struct HASHHDR
Size keysize; /* hash key length in bytes */
Size entrysize; /* total user element size in bytes */
long num_partitions; /* # partitions (must be power of 2), or 0 */
long ffactor; /* target fill factor */
long max_dsize; /* 'dsize' limit if directory is fixed size */
long ssize; /* segment size --- must be power of 2 */
int sshift; /* segment shift = log2(ssize) */
@ -497,8 +495,6 @@ hash_create(const char *tabname, long nelem, HASHCTL *info, int flags)
/* ssize had better be a power of 2 */
Assert(hctl->ssize == (1L << hctl->sshift));
}
if (flags & HASH_FFACTOR)
hctl->ffactor = info->ffactor;
/*
* SHM hash tables have fixed directory size passed by the caller.
@ -603,8 +599,6 @@ hdefault(HTAB *hashp)
hctl->num_partitions = 0; /* not partitioned */
hctl->ffactor = DEF_FFACTOR;
/* table has no fixed maximum size */
hctl->max_dsize = NO_MAX_DSIZE;
@ -670,11 +664,10 @@ init_htab(HTAB *hashp, long nelem)
SpinLockInit(&(hctl->freeList[i].mutex));
/*
* Divide number of elements by the fill factor to determine a desired
* number of buckets. Allocate space for the next greater power of two
* number of buckets
* Allocate space for the next greater power of two number of buckets,
* assuming a desired maximum load factor of 1.
*/
nbuckets = next_pow2_int((nelem - 1) / hctl->ffactor + 1);
nbuckets = next_pow2_int(nelem);
/*
* In a partitioned table, nbuckets must be at least equal to
@ -733,7 +726,6 @@ init_htab(HTAB *hashp, long nelem)
"DIRECTORY SIZE ", hctl->dsize,
"SEGMENT SIZE ", hctl->ssize,
"SEGMENT SHIFT ", hctl->sshift,
"FILL FACTOR ", hctl->ffactor,
"MAX BUCKET ", hctl->max_bucket,
"HIGH MASK ", hctl->high_mask,
"LOW MASK ", hctl->low_mask,
@ -761,7 +753,7 @@ hash_estimate_size(long num_entries, Size entrysize)
elementAllocCnt;
/* estimate number of buckets wanted */
nBuckets = next_pow2_long((num_entries - 1) / DEF_FFACTOR + 1);
nBuckets = next_pow2_long(num_entries);
/* # of segments needed for nBuckets */
nSegments = next_pow2_long((nBuckets - 1) / DEF_SEGSIZE + 1);
/* directory entries */
@ -804,7 +796,7 @@ hash_select_dirsize(long num_entries)
nDirEntries;
/* estimate number of buckets wanted */
nBuckets = next_pow2_long((num_entries - 1) / DEF_FFACTOR + 1);
nBuckets = next_pow2_long(num_entries);
/* # of segments needed for nBuckets */
nSegments = next_pow2_long((nBuckets - 1) / DEF_SEGSIZE + 1);
/* directory entries */
@ -975,7 +967,7 @@ hash_search_with_hash_value(HTAB *hashp,
* order of these tests is to try to check cheaper conditions first.
*/
if (!IS_PARTITIONED(hctl) && !hashp->frozen &&
hctl->freeList[0].nentries / (long) (hctl->max_bucket + 1) >= hctl->ffactor &&
hctl->freeList[0].nentries > (long) (hctl->max_bucket + 1) &&
!has_seq_scans(hashp))
(void) expand_table(hashp);
}

2
src/include/utils/hsearch.h
View File

@ -68,7 +68,6 @@ typedef struct HASHCTL
long ssize; /* segment size */
long dsize; /* (initial) directory size */
long max_dsize; /* limit to dsize if dir size is limited */
long ffactor; /* fill factor */
Size keysize; /* hash key length in bytes */
Size entrysize; /* total user element size in bytes */
HashValueFunc hash; /* hash function */
@ -83,7 +82,6 @@ typedef struct HASHCTL
#define HASH_PARTITION 0x0001 /* Hashtable is used w/partitioned locking */
#define HASH_SEGMENT 0x0002 /* Set segment size */
#define HASH_DIRSIZE 0x0004 /* Set directory size (initial and max) */
#define HASH_FFACTOR 0x0008 /* Set fill factor */
#define HASH_ELEM 0x0010 /* Set keysize and entrysize */
#define HASH_BLOBS 0x0020 /* Select support functions for binary keys */
#define HASH_FUNCTION 0x0040 /* Set user defined hash function */